ByteArrayExtensions.cs

using System.IO;
using System.IO.Compression;

public static class ByteArrayExtensions
{
    public static byte[] Compress(this byte[] raw)
    {
        using (MemoryStream memory = new MemoryStream())
        {
            using (GZipStream gzip = new GZipStream(memory,CompressionMode.Compress, true))
            {
                gzip.Write(raw, 0, raw.Length);
            }
            return memory.ToArray();
        }
    }

    public static byte[] Decompress(this byte[] gzip)
    {
        // Create a GZIP stream with decompression mode.
        // ... Then create a buffer and write into while reading from the GZIP stream.
        using (GZipStream stream = new GZipStream(new MemoryStream(gzip), CompressionMode.Decompress))
        {
            const int size = 4096;
            byte[] buffer = new byte[size];
            using (MemoryStream memory = new MemoryStream())
            {
                int count = 0;
                do
                {
                    count = stream.Read(buffer, 0, size);
                    if (count > 0)
                    {
                        memory.Write(buffer, 0, count);
                    }
                }
                while (count > 0);
                return memory.ToArray();
            }
        }
    }
}